This invention generally relates to interferometry and more particularly to apparatus and methods for interferometrically measuring the position and angular orientation of a body with high accuracy as it travels along a prescribed path, e.g., a nominally straight line in, for example, a process for determining the shape of wavefronts and optical surfaces, such as aspherical surfaces.
There are a number of applications in which an object needs to be scanned accurately along a prescribed trajectory. This is frequently achieved by servo positioning the object using sensors referenced to a mechanical reference structure (or metrology frame). When large motions are required, the metrology frame typically becomes large and hence susceptible to mechanical vibrations, thermal drifts, and other perturbations.
Metrology systems for ultra-precision machines, such as photolithography tools, diamond turning machines, etc., use stable reference frames probed at normal incidence by non-contact gauges, typically laser interferometers. When only small motions are required in one or more degrees of freedom, capacitance gauges or even contacting gauges such as LVDTs may be used. The reference surfaces generally have dimensions similar to those of the travel to be measured in each dimension.
Because of the continuing need for highly precise measuring instruments, it is a primary object of the present invention to provide a compact interferometric motion tracking architecture for precisely measuring the position and angular orientation between two objects undergoing relative motion.
It is another object of this invention to provide method(s) and apparatus for interferometrically measuring relative or absolute distances with high accuracy.
It is another object of the invention to provide method(s) and apparatus for interferometrically measuring slopes, curvatures, and shapes of optics with high accuracy.
It is still another object of the present invention to provide interferometric method(s) and apparatus for high accuracy measurement using redundant, self-checking metrology of the straightness of motion of a moving element along its travel.
Another object of this invention is to provide comparative, self-checking methods for calibrating the straightness of a scanning path in an interferometer.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following description is read in connection with the drawings.